The present invention relates to a cooling system for an electronic apparatus such as a computer, and more particularly to a cooling system having two or more cooling units in which an operation frequency of each cooling unit can be reduced to provide a long life for a compressor in the cooling unit.
In a conventional cooling system having two or more individually adjustable cooling units, a required minimal number of cooling units are operated in order to cool cooling medium supplied to an electronic apparatus to a predetermined temperature, as disclosed in JP-A-2-192564 (corresponding to U.S. Pat. No. 5,201,187). One or more cooling units other than the required minimal ones are left in a stand-by state for spare use. Thus, they are not normally operated unless an abnormality occurs in one of the operating cooling units, in which case one of them is operated as a back-up unit.
In this manner, with a cooling system comprising a plurality of cooling units, Each of the cooling units has a wide cooling range so that a cooling range provided by operating n cooling units may overlap with a cooling range provided by operating n+1 cooling units. In such a case, cooling power corresponding to an amount of heat generated by an electronic apparatus may be generated by either n cooling units or n+1 cooling units. It will be of course understood in this case that the cooling power generated by each cooling unit is lower when n+1 cooling units are operated.
Also, in a compressor provided in each cooling unit for performing a variable control of the cooling power, a speed at which a bearing for converting rotational motion of a motor to a compressing operation is worn is related to the cooling power. More specifically, if an operation frequency of the motor is increased to rotate the motor at a higher rotational speed for enhancing the cooling power, the bearing is worn in a correspondingly shorter time, whereby the useful life thereof becomes shorter. Conversely, if the operation frequency is decreased to rotate the motor at a lower rotational speed so as to lower the cooling power, the wearing speed of the bearing is slowed and accordingly the useful life of the bearing becomes longer. Therefore, for prolonging the useful life of the bearing, the operation frequency of the motor should be deceased to rotate the motor at a lower rotational speed so as to reduce the cooling power of the cooling unit as much as possible.
However, the above-stated prior art does not consider a situation wherein cooling power corresponding to an amount of heat generated by an electronic apparatus may be generated by either n cooling units or n+1 cooling units, and does not either disclose that an operation with n+1 cooling units is selected for this situation so as to provide a more than sufficient cooling power for each cooling unit. It does not either consider that the cooling power generated by each cooling unit is reduced to prolong the useful life of the bearing in the compressor.
For this reason, the conventional cooling system always drives only a required minimal number of cooling units, so that each of the cooling units must continue generating higher cooling power with a few margin kept in reserve. Thus, the prior art presents a problem that the motor in the compressor always continues rotating at a high rotational speed so that the bearing of the compressor is worn fast, whereby the useful life of the bearing is shortened.